Peterson M., Guterl J., Nichols R., et al. (2015). ACVIM Forum. Indianapolis, Indiana: 557.
In human patients, measurement of serum TSH is commonly used as a first-line discriminatory test of thyroid function, even before evaluation of serum T4, T3, or free T4 (FT4). Recent reports indicate that the canine TSH assay (Immulite Canine TSH, Siemens) can be used to measure feline TSH, and that this may be used to help diagnose or exclude hyperthyroidism in cats suspected of having the disease. Because high circulating T4 suppresses TSH secretion, serum TSH levels should be low in hyperthyroid cats, possibly even before serum T4 or free T4 are clearly elevated. Some have suggested that finding a measurable cTSH in a hyperthyroid cat is so unlikely that this alone can be used to rule out this disease. The purpose of this study was to investigate the usefulness of cTSH measurements in the diagnosis of mild to severe hyperthyroidism and in differentiating hyperthyroid cats (n=786) from healthy cats (n=120) and euthyroid cats suspected of hyperthyroidism but found not to have the disease (n=28). For inclusion, all cats had routine laboratory testing, a complete serum thyroid panel (T4, T3, FT4 by dialysis, TSH), and thyroid scintigraphy. Subsequent calculation of thyroid-to-salivary [T/S] and thyroid-to-background [T/B] ratios were used to confirm or exclude hyperthyroidism, as previously described (Thyroid scintigraphy findings in 2,096 cats with hyperthyroidism. Vet Radiol Ultrasound, in press). Median serum concentrations of T4, T3, and FT4 in the 786 hyperthyroid cats (7.9 μg/dl, 151 ng/dl, 100 pmol/L, respectively) were significantly higher (P<0.001, Kruskal–Wallis test) than median concentrations in the 28 euthyroid cats with suspected thyroid disease (2.2 μg/dl, 54 ng/dl, 38 pmol/L) or the 120 clinically normal cats (2.2 μg/dl, 50 ng/dl, 33 pmol/L). Of the hyperthyroid cats, 742 (94%), 513 (65%), and 757 (96%) had high serum concentrations of of T4, T3, and FT4, respectively; of the 28 euthyroid cats, 2 (7%,), 1 (3.5%), and 5 (18%) had falsely high values of T4, T3, or FT4, respectively. The calculated sensitivity of T4, T3, and FT4 as a diagnostic test for hyperthyroidism was 94%, 65%, and 96%, respectively, whereas the specificity of each test was 93%, 96%, and 82%, respectively. Median serum TSH concentration in the 786 hyperthyroid cats (0.03 ng/ml) was significantly (P<0.001) lower than median values in the normal cats (0.05 ng/ml) and the euthyroid cats with suspected thyroid disease (0.06 ng/ml). No significant difference existed between the median concentrations of T4, T3, free T4, or TSH in clinically normal cats and the values in the suspect cats. Only 14 (1.8%) hyperthyroid cats had measurable TSH values, whereas 98.2% had TSH concentrations at or below the level of quantification (<0.03). In contrast, TSH concentrations were detectable in 71 (59%) of the 120 normal cats and in 24 (86%) of the 28 euthyroid cats. Overall, the sensitivity of serum TSH as a diagnostic test for hyperthyroidism was 98.2%, but the test specificity was only 49.3%. Of the 14 hyperthyroid cats with detectable serum TSH values, 10 had mild disease (T4 <6 μg/dl) and 4 had moderate hyperthyroidism (T4≥ 6-12 μg/dl); none of the cats with severe thyroid disease had measurable TSH values. Prevalence of detectable serum TSH values in cats with mild hyperthyroidism (4.4%) was significantly (P=0.016) higher than the prevalence in cats with moderate disease (1.2%). In conclusion, serum TSH concentrations are suppressed in over 98% of hyperthyroid cats, but values are indeed measurable in a few cats with mild to moderate hyperthyroidism. Although most clinically normal cats and euthyroid cats suspected of having thyroid disease will have detectable TSH concentrations, about 40% of the euthyroid cats in this study also had serum concentrations that were below the level of quantification. Overall, serum TSH determination represents a highly sensitive but poorly specific test for diagnosis of feline hyperthyroidism when used in combination with T4 and FT4 determinations.